1. Technical Field of the Invention
The present invention relates generally to a gas concentration measuring apparatus for measuring the concentration of gases which may be employed in an air-fuel ratio control system for automotive vehicles, and more particularly to a gas concentration measuring apparatus designed to minimize an error component contained in an output thereof.
2. Background Art
Recently, NOx sensors designed to measure the concentration of nitrogen oxide (NOx) contained in exhaust emissions of automotive engines are proposed and put into practical use.
As one of such NOx sensors, a gas sensor is known which is designed to measure the concentrations of NOx and O2 contained in exhaust gasses of the engine simultaneously. This type of gas sensor includes a pump cell for decomposing or ionizing oxygen molecules contained in exhaust gasses to measure the concentration of O2 and a sensor cell for decomposing NOx in the oxygen-decomposed exhaust gasses to measure the concentration of NOx. The measurement of the concentration of each of NOx and O2 is achieved by applying a given voltage to a corresponding one of the pump cell and the sensor cell to induce flow of current as a function of one of the concentrations of NOx and O2. The current is outputted from the gas sensor and converted into a voltage signal which is, in turn, used in, for example, an engine control unit of the vehicle.
The above gas sensor, however, has the drawback in that the amount of current flowing through the cell sensor as a function of the concentration of NOx is extremely small, so that it apt to interfere with electrical noises, resulting in a failure in measuring the concentration of NOx accurately. Specifically, when the concentration of NOx is within 0 to 2000 ppm, a current output from the sensor cell is as little as 5 to 10 xcexcA. Therefore, in the case where the gas sensor is used in an engine control system of an automotive vehicle, signal outputs from peripheral electrical devices are added to an output of the gas sensor as noises which will produce an error in measuring the concentration of NOx.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a gas concentration measuring apparatus designed to minimize an error component contained in an output of the apparatus.
According to one aspect of the invention, there is provided a gas concentration measuring apparatus. The gas concentration measuring apparatus includes: (a) a gas concentration sensor outputting a signal as a function of concentration of a given component of gasses; (b) a signal processing circuit processing the signal outputted from the gas concentration sensor to produce a voltage signal indicative of the concentration of the given component of the gasses; and (c) a conductor electrically connecting the gas concentration sensor and the signal processing circuit for transmission of the signal. The conductor has a length which is determined as a function of a level of the signal outputted from the gas concentration sensor. The weaker the level of the signal is, the shorter the length of the conductor.
In the preferred mode of the invention, a connector is provided which connects the gas concentration sensor with an external device. The connector has disposed therein the signal processing circuit.
An impedance measuring circuit is provided which measures the impedance of a sensor element of the gas concentration sensor. The impedance measuring circuit is integrated in a single unit together with the signal processing circuit.
A heater and a heater control circuit are provided. The heater heats up a sensor element of the gas concentration sensor. The heater control circuit controls a power supply to the heater. The heater control circuit is integrated in a single unit together with the signal processing circuit.
The gas concentration measuring apparatus may be mounted in a vehicle to measure, for example, the concentrations of O2 and NOx contained in exhaust emissions of a combustion engine for use in an air-fuel ratio control. The weaker the level of the signal is, the shorter a distance between the gas concentration sensor and the signal processing circuit for minimizing addition of electrical noises produced by electrical devices mounted in the vehicle to the signal outputted from the gas concentration sensor.
The gas concentration sensor includes a first cell responsive to application of a voltage to discharge oxygen contained in the gasses outside the gas concentration sensor, producing a first electric current as a function of concentration of the discharged oxygen and a second cell responsive to application of a voltage to produce a second electric current as a function of concentration of a specified gas component contained in the gasses from which the oxygen is discharged by the first cell.
The signal processing circuit has a function of compensating for a unit-to-unit variation in characteristic of the gas concentration sensor.
The signal processing circuit corrects an output characteristic of the gas concentration sensor so as to agree with a desired one.
The impedance measuring circuit has a function of compensating for a unit-to-unit variation in characteristic of the gas concentration sensor.
The impedance measuring circuit produces an impedance signal indicative of the impedance of the sensor element of the gas concentration sensor and corrects the impedance signal so as to eliminate a variation in the impedance signal caused by the unit-to-unit variation in characteristic of the gas concentration sensor.
The heater control circuit connects with the heater through a power supply conductor for supplying the power to the heater. The heater control circuit has a function of minimizing an error component caused by a resistance value of the power supply conductor.
The signal processing circuit, the impedance measuring circuit, and the heater control circuit are formed on a bare chip mounted on a ceramic substrate.
According to another aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas concentration sensor outputting a signal as a function of concentration of a given component of gasses; (b) a signal processing circuit processing the signal outputted from the gas concentration sensor to provide a voltage signal indicative of the concentration of the given component of the gasses; and (c) a connector having disposed therein the signal processing circuit, the connector having a first end coupled to the signal processing circuit and a second end providing electrical connection with an external device to transmit the voltage signal to the external device.